In fluid bed systems operating a low superficial velocity, gas voidages such as bubbles tend to form which decreases contacting between the gas and the solid phase. In certain situations, internals such as chevrons, subway grating, structure packing or the like are used to break the bubbles and/or prevent formation of bubbles so as to decrease or eliminate the negative impact of inadequate solid/gas phase contact.
In a typical propane dehydrogenation process, internals are desired in the catalyst conditioning zone, the combustor, the reactor stripper, and the reactor itself Subway grating is an excellent choice as it breaks large bubbles into small bubbles while not restricting radial motion the bed.
At a given gas velocity and flux through a given internal that blocks some of the vessel open area, the bed will flood which will not allow solids to backmix to lower levels and will result in excessive entrainment to the top level of the internal structure. Therefore, the open area and associated gas velocities must be controlled within strict limits of 0.1 ft/s-10 ft/s. Based on the solids flux and volumetric gas flow rate, the minimum open area can be calculated as to avoid flooding. Further, the spacing of internals such as subway grating must be set to avoid the streaming of gas up one side of the structure. Finally, due to the large forces and metal movements arising from high temperatures, a unique mechanical design must be used to account for such movement without causing excessive stress on the vessel or the internals.